The present invention relates to the field of secondary radar receivers. Secondary radar is used to obtain, from co-operating carrier vehicles equipped with transponders, coded information elements on the identity of the carrier and other information.
The transponders emit replies upon receipt of interrogation transmissions and may also spontaneously transmit in a mode of operation with selective addressing, called mode S operation. Secondary radar must therefore be provided with means enabling it to recognise those responses, among all the responses received, that are responses to its own interrogations, and having detected them, to decode and validate the code of the response.
As standardised by the International Civil Aviation Organisation (ICAO), a response in mode S is constituted by a train of pulses emitted on a carrier frequency of 1090 MHz, Each train of pulses comprises a preamble and a data block.
The preamble has four identical pulses with a nominal duration of 0.5 microseconds each. The first two pulses and the last two pulses are separated from each other by 0.5 microseconds. The first pulse and the third pulse are separated from each other by 3.5 microseconds.
The message or data block may be short or long. A short message message has 56 pulses of 0.5 microseconds each, and when it is long, it has 112. The modulation of the message is done by the position of the pulses that may be at the first or second half of 1 microsecond intervals. Some of the pulses merge and are then nominally one microsecond long, for example when a “01” code sequence occurs in the SSR response message. The first of these intervals begins 8 microseconds after the start of the preamble.
A secondary response is formed by a pulse train. Each pulse has a leading edge such that, in 50 nanoseconds, a power level representing 90% of the maximum level is reached. This pulse comprises a plateau corresponding to the power level and a decreasing edge. The time difference between the point of the leading edge and the point of the trailing edge having a power level equal to 50% of the maximum power of the pulse is 0.45 microseconds plus or minus 0.1 microseconds.
There is a surveillance technique, related to Mode S, which is known as ADS-B, where an aircraft may spontaneously transmit information such as position, that it has measured using satellite or other navigation means, coded into a set of Mode S messages.
It is expected that Mode S responses, spontaneous Mode S transmissions, surveillance modes such as ADS-B, and other uses of the 1090 MHz band will lead to a substantially increased number of messages in the future. There are currently no satisfactory mechanisms to ensure that only one message will be being received at any one time by the receiver. The term ‘garbling’ is used to refer to instances where more than one message is present at a receiver at anyone time. A garbled message is one where, during the time it is received, one or more other messages are present, coincident, or starting and stopping during the time period of the garbled message. Using conventional decoding methods, this will cause an increasingly high proportion of messages to be unreadable and limit the capacity of the system. This is because the decoding methods currently deployed are known have a limited ability to deal with the consequences of garbling.
The receiving processes employed for receiving the SSR signals are typically to receive the signal's 1090 MHz reply and to mix down to an intermediate frequency before envelope-detecting the signal, providing a voltage signal that represents the logarithm of the envelope amplitude versus time. Monopulse SSR radars also provide a signal that represents the angular deflection of a signal, relative to the centre of the main beam that indicates the direction of arrival, provided that there is only one signal being received. The noise bandwidth of the voltage channel is typically 8 MHz, and matched filtering techniques for reception are not commonly used.
In a well known technique for decoding Mode S signals, using the receiving processes as described above, the position of the preamble bits are detected and used to set a sampling position, two samples are made every bit period as shown for an example bit in FIG. 1. By comparing the magnitudes of the samples a decision can be made as to whether the bit is a 0 or a 1. A large ratio in magnitudes signifies a high probability of a correct decision. This method is susceptible to interference since garbling occurring at the time of sampling will often cause a wrong decision to be made. This method will not be suitable for Mode S with ADS-B except in very light traffic densities.
A new algorithm in the public domain has been defined in order to improve the chances of decoding a signal in the presence of garbling, The algorithm is described in Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance-Broadcast (ADS-B) and Traffic Information Services-Broadcast (TIS-B), RTCA DO-260A, Appendix 1, RTCA Inc 2003. The principle of operation of this algorithm is shown in FIG. 2.
A higher sampling rate is employed than the common techniques, which generate 10 samples within the 1 microsecond bit period. The samples are categorised into one of 4 amplitude bands determined by analysis of the Mode S preamble, this is coded as 2 bits. From the 20 bits so formed a look-up table with 2 to the power 20 combinations is accessed to read the bit state and the bit confidence levels.
This technique is improved when compared with the original technique described in FIG. 1, because it uses more of the information in the signal with which to base its decision, and so a higher degree of garbling can be tolerated before bit errors are made. However it does not allow the separation of the required signal from the garbling signal.
Other known art is described in U.S. Pat. No. 6,094,169, which is a multi-lateration approach to measuring time differences from replies at multiple receive stations, U.S. Pat. No. 5,0633,86, which describes a method for multipath reduction and garbling reduction utilizing histogramming techniques, and U.S. Pat. No. 5,406,288 which describes a sampling and synchronisation method to reduce the probability of a garbled message.
In order to separate garbled signals, superresolution approaches have been suggested. WO02082121 describes the use of the slight frequency difference between the received carrier frequencies of garbled signals. This frequency difference is comprised of the errors in setting downlink carrier frequency, and also the Doppler shift of received signals. The downlink transmitter frequency reference source will typically be a crystal oscillator, and its accuracy will be in the order of one part in one million or worse The Doppler shift will be approximately in the range +/−1 kHz for civilian air traffic at 1090 MHz. The observation time for a single message of 120 microseconds will require very high signal to noise ratios for signals separated by one kilohertz or so, and in particular closely spaced aircraft in an airlane will tend to have highly similar Doppler shifts.
So it will be seen that previously known methods of decoding a 1090 MHz Mode S SSR transmission are limited to allowing a limited degree of decoding to be made
in the presence of garbling, but not in general allowing the separation of signals.